How to: Hook Up a Delegate Using Reflection

When you use reflection to load and run assemblies, you cannot use language features like the C# += operator or the Visual Basic AddHandler statement to hook up events. The following procedures show how to hook up an existing method to an event by getting all the necessary types through reflection, and how to create a dynamic method using reflection emit and hook it up to an event.

Note:

For another way to hook up an event-handling delegate, see the code example for the AddEventHandler method of the EventInfo class.

To hook up a delegate using reflection

Load an assembly that contains a type that raises events. Assemblies are usually loaded with the Assembly.Load method. To keep this example simple, a derived form in the current assembly is used, so the GetExecutingAssembly method is used to load the current assembly.

Get a Type object representing the type, and create an instance of the type. The CreateInstance(Type) method is used in the following code because the form has a default constructor. There are several other overloads of the CreateInstance method that you can use if the type you are creating does not have a default constructor. The new instance is stored as type Object to maintain the fiction that nothing is known about the assembly. (Reflection allows you to get the types in an assembly without knowing their names in advance.)

Get an EventInfo object representing the event, and use the EventHandlerType property to get the type of delegate used to handle the event. In the following code, an EventInfo for the Click event is obtained.

Create an instance of the delegate, using the CreateDelegate method. This method is static (Shared in Visual Basic), so the delegate type must be supplied. Using the overloads of CreateDelegate that take a MethodInfo is recommended.

Get the add accessor method and invoke it to hook up the event. All events have an add accessor and a remove accessor, which are hidden by the syntax of high-level languages. For example, C# uses the += operator to hook up events, and Visual Basic uses the AddHandler statement. The following code gets the add accessor of the Click event and invokes it late-bound, passing in the delegate instance. The arguments must be passed as an array.

To generate an event handler at run time by using a dynamic method

Event-handler methods can be generated at run time, using lightweight dynamic methods and reflection emit. To construct an event handler, you need the return type and parameter types of the delegate. These can be obtained by examining the delegate's Invoke method. The following code uses the GetDelegateReturnType and GetDelegateParameterTypes methods to obtain this information. The code for these methods can be found in the Example section later in this topic.

It is not necessary to name a DynamicMethod, so the empty string can be used. In the following code, the last argument associates the dynamic method with the current type, giving the delegate access to all the public and private members of the Example class.

Generate a method body. This method loads a string, calls the overload of the MessageBox.Show method that takes a string, pops the return value off the stack (because the handler has no return type), and returns. To learn more about emitting dynamic methods, see How to: Define and Execute Dynamic Methods.

using System;
using System.Reflection;
using System.Reflection.Emit;
using System.Windows.Forms;
class ExampleForm : Form
{
public ExampleForm() : base()
{
this.Text = "Click me";
}
}
class Example
{
publicstaticvoid Main()
{
Example ex = new Example();
ex.HookUpDelegate();
}
privatevoid HookUpDelegate()
{
// Load an assembly, for example using the Assembly.Load // method. In this case, the executing assembly is loaded, to // keep the demonstration simple. //
Assembly assem = Assembly.GetExecutingAssembly();
// Get the type that is to be loaded, and create an instance // of it. Activator.CreateInstance has other overloads, if // the type lacks a default constructor. The new instance // is stored as type Object, to maintain the fiction that // nothing is known about the assembly. (Note that you can // get the types in an assembly without knowing their names // in advance.) //
Type tExForm = assem.GetType("ExampleForm");
Object exFormAsObj = Activator.CreateInstance(tExForm);
// Get an EventInfo representing the Click event, and get the // type of delegate that handles the event. //
EventInfo evClick = tExForm.GetEvent("Click");
Type tDelegate = evClick.EventHandlerType;
// If you already have a method with the correct signature, // you can simply get a MethodInfo for it. //
MethodInfo miHandler =
typeof(Example).GetMethod("LuckyHandler",
BindingFlags.NonPublic | BindingFlags.Instance);
// Create an instance of the delegate. Using the overloads // of CreateDelegate that take MethodInfo is recommended. //
Delegate d = Delegate.CreateDelegate(tDelegate, this, miHandler);
// Get the "add" accessor of the event and invoke it late-// bound, passing in the delegate instance. This is equivalent // to using the += operator in C#, or AddHandler in Visual // Basic. The instance on which the "add" accessor is invoked// is the form; the arguments must be passed as an array. //
MethodInfo addHandler = evClick.GetAddMethod();
Object[] addHandlerArgs = { d };
addHandler.Invoke(exFormAsObj, addHandlerArgs);
// Event handler methods can also be generated at run time, // using lightweight dynamic methods and Reflection.Emit. // To construct an event handler, you need the return type // and parameter types of the delegate. These can be obtained // by examining the delegate's Invoke method. // // It is not necessary to name dynamic methods, so the empty // string can be used. The last argument associates the // dynamic method with the current type, giving the delegate // access to all the public and private members of Example, // as if it were an instance method. //
Type returnType = GetDelegateReturnType(tDelegate);
if (returnType != typeof(void))
thrownew ApplicationException("Delegate has a return type.");
DynamicMethod handler =
new DynamicMethod("",
null,
GetDelegateParameterTypes(tDelegate),
typeof(Example));
// Generate a method body. This method loads a string, calls // the Show method overload that takes a string, pops the // return value off the stack (because the handler has no // return type), and returns. //
ILGenerator ilgen = handler.GetILGenerator();
Type[] showParameters = { typeof(String) };
MethodInfo simpleShow =
typeof(MessageBox).GetMethod("Show", showParameters);
ilgen.Emit(OpCodes.Ldstr,
"This event handler was constructed at run time.");
ilgen.Emit(OpCodes.Call, simpleShow);
ilgen.Emit(OpCodes.Pop);
ilgen.Emit(OpCodes.Ret);
// Complete the dynamic method by calling its CreateDelegate // method. Use the "add" accessor to add the delegate to// the invocation list for the event. //
Delegate dEmitted = handler.CreateDelegate(tDelegate);
addHandler.Invoke(exFormAsObj, new Object[] { dEmitted });
// Show the form. Clicking on the form causes the two // delegates to be invoked. //
Application.Run((Form) exFormAsObj);
}
privatevoid LuckyHandler(Object sender, EventArgs e)
{
MessageBox.Show("This event handler just happened to be lying around.");
}
private Type[] GetDelegateParameterTypes(Type d)
{
if (d.BaseType != typeof(MulticastDelegate))
thrownew ApplicationException("Not a delegate.");
MethodInfo invoke = d.GetMethod("Invoke");
if (invoke == null)
thrownew ApplicationException("Not a delegate.");
ParameterInfo[] parameters = invoke.GetParameters();
Type[] typeParameters = new Type[parameters.Length];
for (int i = 0; i < parameters.Length; i++)
{
typeParameters[i] = parameters[i].ParameterType;
}
return typeParameters;
}
private Type GetDelegateReturnType(Type d)
{
if (d.BaseType != typeof(MulticastDelegate))
thrownew ApplicationException("Not a delegate.");
MethodInfo invoke = d.GetMethod("Invoke");
if (invoke == null)
thrownew ApplicationException("Not a delegate.");
return invoke.ReturnType;
}
}